1. Field of the Invention
The present invention relates to an SDH 2-Fiber Ring (synchronous digital hierarchy 2-fiber ring) optical multiplexing device, and more particularly to an SDH 2-Fiber Ring having a function which selectively limits the number of protection channels when a communications fault occurs and increases the usable bandwidth during normal operation.
2. Description of the Related Art
At present, in 2-fiber ring multiplexing devices having a protection function, for all traffic, the transmitting-side node transmits the same data on both the clockwise and the counterclockwise transmission paths, and the receiving-side node uses whichever data is received normally.
In conventional nodes, optical signals are received from the optical fiber transmission path and pass through an opto-electrical converter Which converts the optical signal to an electrical signal, the electrical signal then passing through an electro-optical converter which converts it to an optical signal to be sent onto the optical transmission path, the signals thus passing through the opto-electrical and electro-optical converters. An ADD/DROP multiplexing section extracts time slot signals addressed to the node in question from each of the clockwise and counterclockwise signals, and also inserts time slot signals intended for the transmission destination. A switching section (SW) which selects and outputs the signals of the received signals that were received normally, and an input/output interface section which serves as the interface with an external device such as 2-Mb/s digital dedicated line telephone exchange.
However, in a transmitting-side node such as described above, because the same data is sent over both the clockwise and the counterclockwise ring transmission paths, if the time slots for both the clockwise and the counterclockwise path between a given pair of connected nodes are the same time slot TS#K, for example, the result will be that all TS#K time slots used between each node of the ring network will be reserved.
As a result, when the number of nodes in a conventional ring became large, there was the problem of a great reduction in the number of time slots assigned between each of the nodes.
For example, if 63 time slots are assigned approximately evening between each of the nodes in a ring, the number of time slots that is usable for each connected node pair (channel) with respect to this number of nodes in case of four nodes (A, B, C, and D) is the number of connection pair between the nodes, that is, the six connections node-A-node-B, node-A-node-C, node-A-node-D, node-B-node-C, node-B-node-D, and node-C-node-D. Therefore, in this case the number of time slots that is usable between each of these connection pairs is 63/6, which would be 10 or 11.